At the present time, most electrical power is generated by relatively large power plants operating on fossil or nuclear fuels or hydroelectric power plants for which relatively few suitable locations exist naturally or can be developed. The distribution of such facilities and the variations in demand for power in different geographic regions requires distribution of generated alternating current (AC) power over a network of power transmission lines referred to as a grid. The voltage at which AC power is distributed is generally chosen in accordance with the length of various links since more power can be transmitted at higher voltages and lower currents with reduced losses for a given cross-sectional size of conductor. Lower voltages for shorter links can be readily develop from high voltage AC power through use of transformers.
In recent years, however, there has been great interest in so-called renewable energy and power sources such as solar power and wind power. In the case of solar power, in particular, the power is generated as a substantially constant direct current (DC) voltage from an array of photovoltaic cells, when in operation. In the case of wind power, while wind turbines can be designed to general AC power, their speed and hence the frequency of the alternating voltage produced is subject to wide variation and thus very difficult to synchronize with a power distribution grid. Therefore, wind turbines are generally designed to deliver DC power as well.
It is also characteristic of renewable energy sources that power cannot be continuously generated. Therefore, some facility for energy storage is generally provided, usually as batteries in which energy is stored through a reversible chemical reaction. For that reason, such energy is input into and recovered from such batteries as DC power.
Many solar and wind power generation installations are built without having any local loads and are designed to deliver power only to the power distribution grid. In these types of installations only a suitable inverter device (e.g. a unidirectional Dc to AC power converter) is necessary to develop and synchronize AC power for delivery to the power distribution grid. Other installations may be built for a local load such as for a residence and may or may not be connected to deliver excess power to the power distribution grid or to obtain power from the grid when local renewable resource power generation and (locally stored power, if available) is insufficient for the loads that may be present at a given time. Between these types of installation, some renewable resource power generation systems are developed as so-called nanogrids where there may be numerous types of loads and where power can be input to the nanogrid from the power distribution grid and excess locally generated power can be output to the grid. In these latter types of renewable resource power generation system, a bi-directional power converter capable of delivering power either to or from the grid is required.
So-called full bridge or H-bridge power converters have drawn interest as both inverters and possible bi-directional operation. However, when such a circuit is used in a rectification mode for delivery of power from the grid as DC power, a very large capacitor is required for the DC output, referred to as the DC link, in order to adequately suppress voltage ripple. Unless the ripple is adequately suppressed large electrical stresses are placed on devices connected to the DC link which may cause premature aging of components and photovoltaic cells in particular. However such large capacitances must be of electrolytic design and thus are of large volume, which reduces the converter power density, and subject to failure.